1. Field of the Invention
The present invention relates to a radio communication apparatus and a power consumption control method therefor and, more specifically, to a radio communication apparatus used in a CDMA mobile communication system, which has a plurality of fingers and is desired to perform rake reception, and a power consumption control method therefor.
2. Description of the Prior Art
Recently, mobile communication systems such as a system using portable telephones have become widespread. One of the communication schemes used by such mobile communication systems is CDMA (Code Division Multiple Access).
According to CDMA, on the transmitting side, data is spread by using one of predetermined spreading codes which differ depending on the data to be transmitted, and the spread data is transmitted. On the receiving side, the data is obtained by spreading (so-called despreading) the reception signal by using a spreading code identical to the one used on the transmitting side (to be precise, a code complex conjugate to the spreading code on the transmitting side). In such communication based on CDMA, on the receiving side, the peak correlation value of a received signal is found out by shifting the despreading timing, thereby regenerating the signal transmitted from the transmitting side.
In an actual communication environment for a mobile communication system, until a signal from one base station reaches a mobile station, a plurality of paths such as direct waves and reflected waves are present. In CDMA, such multipath signals can be separated from each other to be recognized as data. Therefore, a path diversity arrangement can be used, in which fingers for despreading the respective multipath signals are arranged for the respective paths, and a rake reception section for combining signals from the respective fingers is used.
FIG. 1 is a block diagram showing a conventional demodulation circuit for performing demodulation by despreading in a CDMA mobile station.
Referring to FIG. 1, this demodulation circuit is comprised of a delay profile calculating section 11 for calculating a delay profile, a finger path allocating section 12 for operating fingers 13a and 13b on the basis of the delay profile generated by the delay profile calculating section 11, a finger section 13 constituted by the fingers 13a and 13b for despreading a reception signal, a rake reception section 14 for combining the despreading results as outputs from the fingers 13a and 13b, and a reception data processing section 15 for demodulating an output from the rake reception section 14 and outputting the resultant digital data as a demodulated output signal.
FIG. 1 shows only two fingers, i.e., the fingers 13a and 13b, for the sake of illustrative convenience. However, this circuit may have more fingers in consideration of the number of multipath signals produced.
The reception signal received by the mobile station is subjected to quadrature detection to be demodulated. The I and Q component signals of this quadrature detection output are input to the delay profile calculating section 11. The delay profile calculating section 11 generates a delay profile by calculating the correlation between the signals.
The finger path allocating section 12 searches the delay profile, calculated and generated by the delay profile calculating section 11, for peaks. Path positions are then allocated, as allocated path positions, to the fingers 13a and 13b in decreasing order of power correlation values. In the finger section 13, the fingers 13a and 13b despread signals sent over the allocated paths. The rake reception section 14 rake-combines the resultant outputs. The reception data processing section 15 demodulates the output from the rake reception section 14, and outputs the resultant digital data, which is the demodulation result, as a demodulation output signal.
As described above, in a CDMA mobile station, a demodulation circuit for performing demodulation by despreading determines paths to be allocated to the respective fingers on the basis of the delay profile generated by a delay profile calculating section.
FIG. 2 is a view for explaining an example of the timing at which delay profile calculation processing is performed in a conventional demodulation circuit.
FIG. 2 is a view showing a reception signal, in which the hatched portions represent pilot symbols (to be described later), i.e., known data portions, and the remaining portions represent information data symbol portions.
As shown in FIG. 2, in the conventional demodulation circuit, delay profile calculation is performed for each pilot symbol, and the values obtained by, for example, N calculations, are averaged, thereby obtaining a delay profile to be output from the delay profile calculating section 11 in FIG. 1.
Delay profile calculation should be done when the effective reception path position changes due to, for example, the movement of the mobile station. However, according to the conventional demodulation circuit, as described above, delay profile calculation is performed at a fixed cycle, and the effective reception path position does not always change in this cycle.
For this reason, in the conventional demodulation circuit, delay profile calculation is performed even when it is not necessary, resulting in a waste of power.
If the intervals at which delay profile calculation is performed are simply prolonged to reduce power consumption, delay profile calculation may not be performed even when it is necessary as the effective reception path position changes, resulting in a demodulation failure.
As communication terminals, like current portable telephone terminals, are required to become smaller in size with longer periods of use, increases in power consumption pose a serious problem. Demands have therefore been arisen for portable telephone terminals capable of reducing power consumption while maintaining good reception characteristics.